1. Field of the Invention
The present invention relates to a driving method of an active matrix display device, which has a switching element in each pixel and a memory for storing a video signal inputted to each pixel. In particular, the invention relates to a driving method of a display device where gray scales are expressed by controlling the light-emission period of each pixel.
2. Description of the Related Art
As a driving method of a display device, such a driving method of a display device has been proposed that gray scales are expressed by dividing one frame period into multiple sub-frame periods and selecting light emission or non-light emission of each pixel by inputting a video signal thereto in each sub-frame period (hereinafter also referred to as a time-division gray scale display) (see Patent Document 1).
For example, one frame period is divided into first to third sub-frame periods, and the ratio of the (light-emission length of the first sub-frame period): (light-emission length of the second sub-frame period):(light-emission length of the third sub-frame period) is set to satisfy 20:21:22. Here, the light-emission length of each sub-frame period corresponds to a period in which a pixel selected for light emission emits light in each sub-frame period. By inputting a video signal into each pixel in each of the first to third sub-frame periods to select light emission or non-light emission of the pixel, 8 gray scales can be expressed.
A display device that performs time-division gray scale display has a panel including multiple pixels and a driver circuit for inputting video signals into the multiple pixels, and a peripheral circuit for inputting signals into the panel. The peripheral circuit generates video signals and timing signals to be inputted into the panel. Based on the signals inputted from the peripheral circuit, the panel performs the time-division gray scale display.
The peripheral circuit of the display device that performs time-division gray scale display has a memory and a controller for controlling the memory. The controller writes (stores) video signals (hereinafter also referred to as source video signals) inputted to the display device into the memory, and reads out the written (stored) video signals to be inputted into the panel. In order to perform time-division gray scale display, it is necessary that video signals are read out from the memory in each sub-frame period. That is, the read operation of video signals from a memory is required to be synchronous with each sub-frame period. On the other hand, source video signals are inputted into the display device independently of the sub-frame periods. That is, the write operation of source video signals into the memory is asynchronous with each sub-frame period.
[Patent Document 1]
Japanese Patent Laid-Open No. 2001-5426
In a peripheral circuit of a display device that performs time-division gray scale display, the write timing of source video signals into a memory is asynchronous with the read timing of video signals from the memory. Therefore, such a method has been adopted that two memories (single-port memories) are provided in the peripheral circuit so that a video signal stored in one memory is read out while a source video signal is written into the other memory. This method requires two memories and a circuit for controlling the write/read operation of video signals to/from the two memories in the peripheral circuit, which results in the complex configuration of the peripheral circuit and larger size of the display device.
Alternatively, there is a method of using one dual-port memory instead of providing two memories in the peripheral circuit. In the dual-port memory, the write operation of source video signals and the read operation of video signals can be performed independently of each other. That is, the read operation of video signals that are written into a dual-port memory can be performed concurrently with the write operation of source video signals into the dual-port memory. However, since the same memory area is used for writing source video signals and reading out video signals in concurrently performing the write operation of source video signals and the read operation of video signals to/from the dual-port memory, signals written into the memory and signals read out from the memory are mixed with each other. Therefore, there is a problem in that video signals cannot be accurately inputted into the panel, which results in the low display quality of images.